1. Field of the Invention
The present invention relates to alignment between two objects, more particularly to wafer/mask alignment in lithographic systems and even more particularly to alignment of lithographic elements in semiconductor manufacturing systems.
2. Description of Related Art
In order to use a wafer stepper to expose images with an x-ray source, the mask and the wafer must be held within close proximity to each other, e. g. 40 micrometers or less apart. Therefore, wafer stepper site by site alignment of circuit levels is performed with no more than a limited distance between the mask plane and the wafer plane. This constraint forces standard, dual focus alignment schemes to receive signals from the two planes, simultaneously.
In one embodiment of such prior alignment systems, the alignment objective lens views the wafer marks through the mask and then refocusses to view the mask marks. Note that the wafer is located in the background as illustrated by FIGS. 1A and 1B. Each of FIGS. 1A and 1B shows a wafer with a mask above it, an objective lens piece above it, plus a CCD for sensing the radiation from the mask and wafer through the objective lens. In each drawing, the solid lines represent the light which is in focus and the dotted lines represent the light which is from the background. FIG. 1A shows the objective lens focussed for viewing an alignment mark on the mask. The solid lines represent the mask mark image. The dotted lines represent the wafer background. In FIG. 1B, which shows the objective lens focussed for viewing the wafer, the solid line represents the wafer mark image and the dotted lines represent the background from the mask. Although the space (proximity gap) between the mask and the wafer is great enough to cause blurring of the defocussed image, the background is very strongly visible and can lead to offsets and to a reduced contrast ratio in the captured (detected) alignment mark image. The background can take the form of reflected or diffracted optical stray light.
In short the general problem to be overcome includes the alteration and/or corruption of the wafer alignment target or mask alignment target in a proximity alignment system caused by optical stray light from a background object in close vertical proximity to the viewing object.
U.S. Pat. No. 4,577,968, commonly assigned, of Makosch for "Method and Arrangement for Optical Distance Measurement" describes a method for determining distances optically where the phase shift of two perpendicularly polarized light beams upon diffraction from an optical grating is determined by directing those beams at an optical grating associated with an object whose displacement is to be measured to cause a phase shift between the two light beams. The light beams, of equal order, are combined after the diffraction from the optical grating. The phase shift caused by the diffraction from the grating is used as a measure of the displacement of the object. Alignment between two objects can be determined in a similar way. If there is perfect alignment of the gratings on the two objects, then there will be no phase difference between the diffracted light from the two objects' gratings. If there is a misalignment, then a signal is generated indicating the degree of misalignment.
In U.S. Pat. No. 4,779,001 of Makosch for "Interferometric Mask-Wafer Alignment" the patent states as follows: "An optical transmission grating is provided in the mask and illuminated by a collimated light beam, e.g. a laser. Two symmetrical diffraction orders, e.g. the +/-1 orders, are then focussed by the imaging system in one common spot on the optical grating that is provided on the wafer and has the same grating constant as the image of the mask grating. The two incident diffraction orders are diffracted a second time at the wafer grating to return along the optical axes and to be deflected by a semi-transparent mirror to a photo detector whose output signal is evaluated for the relative phase difference of the two diffracted beams. For that purpose the phase of the two beams that are diffracted at the mask is changed periodically in three steps, e.g., by a thin oscillating glass plate arranged in series with the mask grating."
To separate X, Y alignment information a lambda/2 film is introduced to beamsplitter 5 in FIG. 2A of Makosch over regions 5a in FIG. 2B. This rotates the polarization of one axis by 90.degree.. X, Y signals are then separated at the detector assembly 6a, 6b, 6c in FIG. 2A due to the orthogonal polarizations of X, Y information.
The current invention differs from this in that the 2-D broadband extended image is formed at the mask and wafer detectors. The polarization rejection eliminates stray optical light and permits mask data to be separated from wafer data, but has no part in decoupling X alignment data from Y alignment data as in Makosch '001.
Advantages of the current invention include process insensitivity due to broadband imaging, proximity plane stray light re]ection, no moving optical components during alignment, simultaneous imaging of the mask and wafer planes and flexibility in mark design.
Makosch '001 uses a pair of gratings on a mask and a wafer and a laser alignment beam. FIG. 2A of Makosch '001 shows a laser beam reflected from a dichroic deflection mirror. The reflected laser beam passes down through an x, y alignment grating on a mask which splits the beam into diffracted orders which traverse a tiltable glass plate and a beam splitter plate to be focussed by linear magnification by an optical system. The tilted glass plate is oriented at 45.degree. with respect to the planes defined by the two pairs of diffracted beams, so that mutual phase differences in the beam pairs are introduced by tilting the glass plate. In the center of the beam splitter, a dichroic mirror totally reflects the partial beams from the wafer mask. The light from the dichroic mirror passes through a polarizing beam splitter to the x and y photo detectors for detection thereby.